1. Field of the Invention
The invention relates generally to a semiconductor device including an insulating film, and more particularly, to improvements in a semiconductor device including an interlayer insulating film mutually insulating a first layer and a second layer of conductor patterns.
2. Description of the Background Art
FIGS. 11A-11D are schematic sectional views for explaining a method of forming an interlayer insulating film by prior art.
Referring to FIG. 11A, a first silicon oxide film 3 is formed over a substrate 1 by the plasma CVD (Chemical Vapor Deposition) method to cover a first conductor pattern 2 formed on the semiconductor substrate 1. In this plasma CVD method, tetraethoxysilane (TEOS: Si (OC.sub.2 H.sub.5).sub.4) which is a kind of silicon alkoxide and oxygen (O.sub.2) gas are reacted with the aid of plasma energy under the pressure of several Torrs at the temperature of 370.degree. C.-420.degree. C. to deposit a first silicon oxide film 3 to the thickness of approximately 0.5 .mu.m. This first silicon oxide film 3 will be referred to as TEOS-PCVD oxide film hereinafter.
Referring to FIG. 11B, a second silicon oxide film is formed over the TEOS-PCVD oxide film 3 by a low pressure CVD method. TEOS and ozone (O.sub.3) are reacted at 370.degree.-420.degree. C. under the pressure of several 10 Torrs in this low pressure CVD method to deposit a second silicon oxide film 4 to the thickness of approximately 0.5 .mu.m. This second oxide film 4 will be referred to as TEOS-LPCVD oxide film hereinafter.
Referring to FIG. 11C, a SOG (Spin-On-Glass) film 5 is formed on the second silicon oxide film 4 by baking at 450.degree. C. for 30-60 minutes. The SOG 5 is partly etched anisotropically to make smooth the upper surface of the TEOS-LPCVD oxide film 4.
Referring to FIG. 11D, silane gas, O.sub.2 gas, and phosphine gas are reacted at the temperature of 400.degree.-450.degree. C. by a low pressure CVD method or an atmospheric pressure CVD method above the smoothed surface to deposit a PSG (Phosphorus Silicate Glass) film 6 to the thickness of approximately 0.2 .mu.m. The second conductor pattern (not shown) is formed over these interlayer insulating films 3-6.
FIG. 12 shows sections of contact holes 7 penetrating the interlayer insulating films 3-6 formed by the prior art shown in FIGS. 11A-11D with second layer conductors 8 connected to the first layer conductor 2 and the semiconductor substrate 1. The contact hole 7 is formed by plasma etching and wet etching. It can be seen from the view that the TEOS-LPCVD oxide film 4 is always exposed at the sidewall of the contact hole 7, whereas the SOG film 5 is only exposed depending on the location of the contact hole. This is because the TEOS-LPCVD oxide film 4 is continuous, whereas the SOG film 5 is patterned. When the second layer conductor 8 is formed by vacuum evaporation, sputtering, etc., the TEOS-LPCVD oxide film 4 and SOG film 5 discharge moisture in the contact hole 7, as shown by arrows in FIG. 12. Consequently, the sidewall of the contact hole 7 may not be completely covered by the second layer conductor 8, leading to some cases where satisfactory interlayer connection is not achieved.
FIG. 13 is a graph showing the infrared absorption of the TEOS-LPCVD oxide film. The abscissa axis represents the wave number (cm.sup.-1), while the ordinate axis represents the transmittance (%). As shown by arrow A, light absorption by Si--OH bonding occurs in the vicinity of the wave number of 3450 cm.sup.-1. This absorption coefficient of approximately 3000 (cm.sup.-1) by this Si--OH bonding is a great value. The TEOS-LPCVD oxide film 4 including many Si--OH bonding will discharge moisture during evaporation process and sputtering process in the vacuum to become the cause of the above mentioned incomplete interlayer connection. In addition, the SOG film 5 includes even a greater number of Si--OH bonds than the TEOS-LPCVD oxide film 4. Accordingly, the interlayer connection will tend to become incomplete if the connection penetrates the SOG film 5.
If the silicon oxide film including Si--OH bonding is annealed, moisture will be discharged to cause shrinking, and cracks are liable to be created in the film. These cracks will degrade the insulation of the silicon oxide film. In practice, under 30 minutes of annealing at 450.degree. C., the TEOS-LPCVD oxide film will shrink 10-15% in thickness, while the SOG film will shrink 20-30% in thickness. This annealing step is inevitably performed after the formation of TEOS-LPCVD oxide film 4 and SOG film 5. For example, annealing is performed to compensate for radiation damage of the transistor formed by ion implantation.
Therefore, the silicon oxide film which shrinks by annealing may not have its film thickness increased to prevent the generation of cracks. If the thickness of the film is increased, stress due to shrinkage will become greater to facilitate the generation of cracks. To prevent the generation of cracks, the thickness of the TEOS-LPCVD oxide film is preferably not more than 0.5 .mu.m, while the thickness of the SOG film is preferably not more than 0.4 .mu.m. This limitation regarding the thickness of the films is one of the reasons for the interlayer insulating films 3-6 to have a multilayer structure. That is to say, it is generally preferable for the interlayer insulating film to have a thickness of 0.8-1.2 .mu.m. If the interlayer insulating film is thinner than 0.8 .mu.m, the breakdown voltage of the interlayer insulating film will become insufficient with the possibility of parasitic capacitance. On the other hand, if the interlayer insulating film is thicker than 1.2 .mu.m, it will become difficult to pattern the interlayer insulting film by etching or to form a contact hole. It is also difficult to form a conductive layer at the edge of the patterned interlayer insulating film or over the high sidewalls of the contact hole. However, if an interlayer insulating film of such thickness is formed by a single silicon oxide film comprising a number of Si--OH bonds, there is the possibility that cracks may be generated in the film.
The major characteristics required in interlayer insulating films are listed as below:
(1) The interlayer insulating film has a smoothed upper surface so that a second layer conductor pattern is easily formed without disconnection on the interlayer insulating film.
(2) Gas discharge from the interlayer insulating film, particularly gas discharge within the contact hole, is minimized to achieve satisfying adhesion of the second layer conductor pattern to the interlayer insulating film.
(3) The interlayer insulating film has satisfying insulation without cracks.
The interlayer insulating films 3-6 of the prior art shown in FIG. 7 at least can not sufficiently satisfy the characteristic of (2), with a further problem of having a complex formation method due to its multilayer structure.